The present invention relates to novel cyclic tetrapeptide derivatives or pharmaceutically acceptable salts thereof, application of said compounds as histone deacetylase inhibitors and MHC class-I molecule expression promoting agents, as well as pharmaceutical compositions that comprise said cyclic tetrapeptide derivatives or pharmaceutically acceptable salts thereof as effective ingredients and which have utility as pharmaceuticals such as anti-cancer agents by taking advantage of the aforementioned histone deacetylase inhibiting or MHC class-I molecule expression promoting action.
Tissue cells of a self inherently express on their cell surface an MHC class-I molecule as an antigen presenting molecule to discriminate externally invading foreign matters and pathogens from themselves and prevent false damage by their immunocytes. The immune system, looking at the MHC class-I molecule, identifies the tissue cells of a self and eliminates them from the target of its attack. On the other hand, cancerized cells or cells infected with cancer viruses, which are originally cells of a self, differ from normal cells of a self in that they produce proteins associated with cancer or proteins derived from the cancer viruses, and antigens derived from these non-self proteins are presented by the MHC class-I molecule. The immunocytes, in particular cytotoxic T cells, can recognize the non-self protein-derived antigens, thereby excluding the cancer cells or cancer virus-infected cells.
It has been reported, however, that in certain kinds of cancer cells or cancer virus infected cells, the expression of the MHC class-I molecule is reduced, so that the aforementioned exclusion mechanism by the immune system is circumvented, causing expansion and enlargement of cancerized tissues as well as prolonged sustention and enlargement of cancer virus infection. In the studies for the purpose of preventing tumorization of the cancerized cells or cancer virus infected cells, some results have been reported suggesting that therapeutic effects may be attained by recovery of the reduced expression of the MHC class-I molecule. For example, Tanaka et al. reported that in cancer cells transformed with adenovirus 12 or spontaneous melanoma, tumorization of these cancer cells may disappear upon enhancing the reduced expression of the MHC class-I molecule through introduction of MHC class-I gene: see Tanaka, R., Isselbacher, K. J., Khoury, G. and Jay, G., Science, 228, 26-30, 1985; Tanaka, K., Gorelik, E., Watanabe, M., Hozumi, N. and Jay, G., Mol. Cell. Biol., 8, 1857-1861, 1988.
By the way, the expression of MHC class-I molecule occurs during the differentiation processes after the growth of the self tissue cells, and the expression of MHC class-I molecule is expected to be promoted by promoting the translation of endogenous proteins in this process. While there are several mechanisms which control the translation of endogenous proteins, one of those which may be considered to play an important role in gene expression is acetylation of histone proteins contained in the nuclear gene chromatins as their structural proteins. Illustratively, chromatin is composed of the basic unit referred to as a nucleosome structure, in which a gene DNA is wound around four core histone octamers. Further, the basic units form higher-order structure. The neighborhood of the N-terminal of the core histone is in the form of a tail rich in basic amino acids and it further encloses the DNA on the aforementioned nucleosome. Lysine residues in the neighborhood of the tail region undergo reversible metabolic turnover of acetylation and are said to be closely involved in the structural control of nucleosome itself or in the transcriptional control through the control of binding with other proteins acting on gene DNA, such as transcriptional factors, silencer proteins and RNA polymerase.
As a demonstration of gene expression control depending on acetylation of histone, it has been reported that higher acetylation of histone promotes the induced expression from genes present in a region of interest while deacetylation forms a transcriptional inactive region called heterochromatin. That is to say, histone which is a structural protein of chromatin and its acetylation are extended over the whole region of the chromosomal gene; nevertheless, it has been suggested that the function of histone greatly affects the expression of a specific gene and, in other words, is involved in the strict control of nuclear signal transmission. An enzyme for acetylating histone is histone acetyl transferase while an enzyme for deacetylating histone is histone deacetylase; these enzymes regulate the kinetic metabolic turnover relating to the level of histone acetylation.
If the action of histone deacetylase is accentuated, proper differentiation of cells or normalization of their morphology is inhibited: however, when the enzyme activity of the histone deacetylase is inhibited, the deacetylation from histone is inhibited and, as a result, high acetylation of histone is caused to induce the gene expression required for differentiation and normalization of cell morphology. This phenomenon has been confirmed to some extent by studies using trichostatin A shown in FIG. 1 or trapoxin analogs shown in FIG. 2, which are enzyme inhibitors of histone deacetylase. In addition, when these inhibitors are allowed to act on cells at higher concentrations, cell cycle inhibition is caused and consequently growth inhibition occurs. Trichostatin A exhibits a non-competitive enzyme inhibiting actions at low concentrations and is a reversible inhibitor; on the other hand, trapoxin analogs exhibit competitive inhibitory actions but are irreversible inhibitors. Further, it has also been reported that enzymatically active subunits of human derived histone deacetylase were purified on an affinity column using K-trap that is a cyclic tetrapeptide compound similar to trapoxin; thus, strong evidence has been given to demonstrate that the cyclic tetrapeptide structure as found in trapoxin and the like forms a selective intermolecular linkage with said enzymatically active subunit.
As stated above, since an enzyme inhibitory substance of histone deacetylase is a drug causing cell differentiation or normal morphogenesis, it may also exhibit a promoting action in the expression of MHC class I molecule which occurs as a step in the process of differentiation; however, no report confirming Gis possibility has been made to date. Accordingly, there is a strong need for search and proposal of histone deacetylase enzyme inhibitory substances that exhibit promoting actions on the expression of MHC class-I molecule in self tissue cells. Further, as stated above, a histone deacetylase enzyme inhibiting substance at a high concentration causes the inhibition of cell cycle and consequently exhibits growth inhibiting action so a need exists for the proposal of a novel anti-cancer agent that is based on the promotion of the MHC class-I molecule expression and which exhibits a combined anti-cancer action due to the contributions of not only the inhibition of tumorization and the exclusion of cancer cells by immune system, but also the cell growth inhibiting action, all being associated with the promotion of MHC class-I molecule expression.
The present invention solves the aforementioned problems and an object of the present invention is to provide a histone deacetylase enzyme inhibiting substance exhibiting a promoting action on the expression of MHC class-I molecule in self tissue cells and to provide a pharmaceutical composition comprising said histone deacetylase enzyme inhibiting substance as an effective ingredient.
To solve the aforementioned problems, the present inventors have eagerly studied and found that trichostatin A or its analogous compound trichostatin C that have a histone deacetylase enzyme inhibiting activity promotes the expression of MHC class-I molecule when they were allowed to act on animal cells and further found that in addition to the said trichostatins, butyric acid and trapoxin A that have the histone deacetylase enzyme inhibiting activity also exhibit the MHC class-I molecule expression promoting activity. Based on these findings, various cyclic tetrapeptide derivatives have been created and these cyclic tetrapeptide derivatives have been found to inhibit the enzyme activity of histone deacetylase reversibly and exhibit the MHC class-I molecule expression promoting activity. Thus, the present invention has been completed.
Accordingly, the present invention relates to a cyclic tetrapeptide derivative represented by any one of the general formula (I): 
the general formula (Ixe2x80x2): 
the general formula (Ixe2x80x3): 
and the general formula (Ixe2x80x2xe2x80x3): 
wherein:
R11, R12, R21 and R22 independently denote a monovalent group selected from hydrogen, linear alkyl groups with 1 to 6 carbon atoms, branched alkyl groups with 3 to 6 carbon atoms, linear xcfx89-aminoalkyl groups with 1 to 5 carbon atoms, branched aminoalkyl groups with 3 to 5 carbon atoms, N-acyl-aminoalkyl groups in which the amino group on said linear or branched aminoalkyl groups is substituted with an acyl group or a halogeno-substituted acyl group that have 3 or less carbon atoms, benzyl group, 4-methoxybenzyl group, 3-indolylmethyl group, (N-methoxy-3-indolyl)methyl group, (N-acyl-3-indolyl)methyl groups having an acyl group with 3 or less carbon atoms as a substituent on the ring-forming nitrogen atom, and methyl group substituted with an aryl group comprising 4 or less rings;
R3 denotes a divalent group selected from linear alkylene groups with 3 or 4 carbon atoms in the chain which may have a branched chain on the chain, linear alkenyleno group with 3 or 4 carbon atoms in the chain which may have a branched chain on the chain, linear alkadienylene groups with 4 carbon atoms in the chain which may have a branched chain on the chain, and those divalent groups in which the branched chain added onto said linear alkylene, linear alkenylene or alkadienylene group form a fused ring structure, as well as those divalent groups in which among the carbon atoms constituting the chained hydrocarbon groups, one of the carbon atoms other than that having a free valence has been replaced with a heteroatom oxygen, sulfur or nitrogen;
R4 denotes a divalent chained hydrocarbon group with 4 to 6 carbon atoms in the chain which may have a branched chain on said chain, or a divalent group in which among the carbon atoms constituting the chained hydrocarbon groups, at least one of the carbon atoms other than that having a free valence has been replaced with a heteroatom oxygen, sulfur or nitrogen;
R4 denotes a divalent chained hydrocarbon group with 4 to 6 carbon atoms in the chain which may optionally have a branched chain on said chain, or a divalent group in which among the carbon atoms constituting the chained hydrocarbon groups, at least one of the carbon atoms other than that having a free valence has been replaced with a heteroatom oxygen, sulfur or nitrogen; and
R5 in the general formulae (Ixe2x80x3) or (Ixe2x80x2xe2x80x3) denotes a methyl group or halogeno-substituted methyl group, or a pharmaceutically acceptable salt thereof; a histone deacetylase inhibitor comprising said cyclic tetrapeptide derivative or pharmaceutically acceptable salt thereof; an MHC class-I molecule expression promoting agent comprising said cyclic tetrapeptide derivative or pharmaceutically acceptable salt thereof as an effective ingredient; as well as a pharmaceutical composition, such as an anti-cancer agent, comprising said cyclic tetrapeptide derivative or pharmaceutically acceptable salt thereof as an effective ingredient.